Although the first and most important epoxy resins are of the glycidyl ether type, other epoxides have been commercially marketed in recent years. Such materials are the cycloaliphatic epoxides. This type of epoxide is a non-glycidyl ether type and is less viscous and less reactive than the typical diglycidyl ether bisphenol A epoxides. It has generally been used as a diluent and plasticizer for the bisphenol A type resins. As such, the cycloaliphatic epoxides have generally been a minor component in standard resinous compositions, comprising up to about 40 but generally less than 10 parts by weight per 100 parts bisphenol A type resin.
Because of their low viscosity, cycloaliphatic epoxides would make useful solventless impregnating varnishes for insulation which relies on mica, in the form of paper, tape or splittings, in large rotating machines. Gel times of these epoxides, with basic curing agents such as amines and basic accelerators such as imidazoles, are relatively slow however. Gel times of these epoxides with acid anhydride curing agents and basic accelerators such as benzyldimethyl aniline and imidazoles, are suitable for commercial applications, but the storage properties are poor.
Another group of epoxides which have recently been marketed are non-ether epoxide resins of the glycidyl ester type. These epoxides can have a low viscosity, also making them useful in solventless impregnating varnishes, but they also have slow gel times and poor pot life with most standard curing agents and accelerators.
There is a need for a primarily cycloaliphatic epoxide or a glycidyl ester epoxide composition, having both commercially acceptable gel times at about 120.degree. to 180.degree. C and superior storage life, coupled with good tensile and electrical properties. This composition is needed, in an inexpensive, low viscosity form, for use as a solventless impregnating varnish for large rotating apparatus insulation.
Such a composition would require a critical formulation of cycloaliphatic epoxide or glycidyl ester epoxide, inexpensive reactive epoxide diluent, and a latent catalyst. The latent catalyst would be required to give a rapid cure between 120.degree. and 180.degree. C; to be completely miscible with the epoxide so that there will be good impregnation and a uniform cure; to provide a composition having a storage life of about 6 months at 25.degree. C; to be a complete substitute for typical curing agents and accelerators and yet not adversely affect the tensile and electrical properities of the cured resin system.
Several latent catalysts and accelerators have appeared on the commercial scene in recent years. Included are boron trifluoride-monoethylamine, quaternary ammonium halides such as benzyltrimethyl-ammonium chloride, stannous octotate, "extra-coordinate" siliconate salts, triethanolamine borate, triethanolamine titanate and various other metal chelates. However, all of these materials have been rejected for one reason or another, some of them being highly exothermic catalysts.
Puchala et al., in U.S. Pat. No. 3,244,670, attempted to solve problems associated with curing epoxy resins. In that patent, epoxides of cyclohexane derivatives were cured with 0.05 to 2.5 wt%, based on epoxide, of an organo-tin halide used as an accelerator, generally in combination with a carboxylic acid anhydride or a phenolic type curing agent. Among the tin compounds suggested were butyl-tin trichloride, dibutyl-tin dichloride, tributyl-tin hydroxide, triphenyltin chloride, phenyl-tin trichloride, diphenyl-tin dichloride, dibutyl-tin oxide and dibutyl-tin sulfide. While the high concentrations of these tin compounds, in conjunction with a carboxylic acid anhydride, effectively cure epoxy resins at 150.degree. C within 60 minutes; the storage life of the longest lived Puchala et al. compositions is only about 20 days at 25.degree. C. In a somewhat similar fashion, Markovitz in U.S. Pat. No. 3,622,524 reacted 20 to 80 wt% of an organo-tin compound, preferably an oxide, with an organic acid or anhydride to form a solid cross-linking agent for epoxide resins used in high voltage stress environments.
Rogers et al., in U.S. Pat. No. 3,759,866, provided a low viscosity, solventless, impregnating composition using a partially ionically bonded quaternary phosphonium salt, as latent accelerator, in a bisphenol A or novolac epoxy system. The system also contained carboxylic acid anhydride curing agent, and diglycidyl ether of neopentyl glycol reactive diluent. While these compositions provided excellent storage stability of about 6 months at room temperature; the gel times of about 75 to 120 minutes at 135.degree. C, and power factor values of about 8.5 to 10% at 125.degree. C (100 .times. tan .delta.), while very good, could be improved. In addition, the use of a carboxylic acid anhydride requires the rigorous exclusion of moisture, otherwise the storage properties might be adversely affected.